The purpose of the proposed project is to investigate whether a previous non-specfic impairment of the lung, as produced by inhalation of inert dusts or noxious gases affect the subsequent immunologic response to inhaled antigenic materials. The animals will be exposed to increasing amount of NO2 and their subsequent response to inhaled antigenic materials, such as pigeon serum or bovine serum albumin will be measured in terms of production of specific serum precipitating antibody and of histological changes in the lung typical for hypersensitivity pneumonitis.